Injured Nadal withdraws from ATP Finals

World number one Rafael Nadal has been forced to withdraw from the ATP Finals due to a knee injury.

A stricken Nadal lost his opening match at the O2 Arena in London, the 16-time grand slam champion upstaged by David Goffin in three sets on Monday.

Nadal - who was already in doubt for the season-ending event after a knee injury meant he had to pull out of the Paris Masters - looked uncomfortable in the 7-6 (7-5) 6-7 (4-7) 6-4 defeat in Group Pete Sampras.

Fellow Spaniard Pablo Carreno Busta has replaced the 31-year-old reigning French and US Open champion as an alternate.

"I'm going to work hard... to keep being at the top and fighting for the most important things. Thank you and Merry Christmas, everyone."